I can’t seem to be able to phrase my question in any simpler way.
Basically, the question refers to Einstein’s theory of relativity, and to an example used to illustrate one of its principles in the text “[Short Words to Explain Relativity](https://www.muppetlabs.com/~breadbox/txt/al.html)”.
I tried to paste the relevant fragment in its entirety, but the bot flagged it as speculative. So here’s a trimmed version I hope will pass the tests:
>We have Bert and Dana. Take a bus, and put Bert on the bus. The bus goes down the road. Dana, she sits here, on the side of the road. He’s in the bus and she’s on her ass. And now take a rock off of the moon, and let it fall at them. It hits the air and cuts in two. The two bits burn, and then land just as Bert and Dana are side by side. One hits the dirt up the road a ways, and one hits down the road a ways. **Dana sees each rock at the same time, but Bert sees one rock and then sees the next rock**.
(continued on the site)
The basic idea is that depending on the point of reference (stationary Dana vs. mobile Bert), the two rocks hit the ground either at the same time or one after the other.
I cannot for the love of me imagine how that would work. Call me naive, but something touching the ground at the same time should look the same to all observers, whether they’re moving or not. So, although I feel stupid asking you to explain something written “in words of four letters or less”, can anybody dumb it down even further?
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In: 143
It all starts with knowing that the speed of light is always the same relative to you. This is different from all other speeds you experience in daily life.
Now imagine you are riding in a moving train car that has a lamp in the exact middle of the car. The lamp flashes on and off, and the light from the lamp hits the back and front of the train car at exactly the same time.
Now imagine your friend is sitting outside by the railroad tracks watching the train pass and he sees the lamp flash. To him, as he faces the train car. the light moves the same speed toward the right (where the front of the car is) as it does toward the left (where the back of the train car is. But of course the train is moving left to right. So your friend sees the left moving light collide with the back of the train care before the right moving light reaches the front of the train car. Thus you and your friend can’t agree on whether the light hit both front and back at exactly the same time or whether it hit the back first and then the front.
So you guys decide to do an experiment. You rig the train car’s front and back to drop a paint bucket on the train track when hit by the light. Then you can measure how far apart the buckets are. It turns out that relativity doesn’t just bend time, it also bends space. So your friend saw a much shorter train car passing by him than the train car you experienced while riding in it.
It’s really difficult to get an intuitive feel for it, but the math all works out.
One common objection is that if events happen in a different order depending on your perspective, then what if you drop a red paper and a blue paper? Which ends up on top if one person sees the red dropped first and the other person sees the blue dropped first? The key to that kind of problem is that how different the time perspectives of the events are depends a lot on how far apart in space the events are. And when you bring the objects back together so that one paper can be on top of the other, there is no longer any disagreement about which paper does what first. When two events happen in the exact same place, everyone agrees on which happened first.
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